Selective catalytic reduction pollution-control system

ABSTRACT

A housing for an ammonia-sensitive component in a selective catalytic reduction pollution-control system. This housing includes a wall and a trap configured to capture gaseous ammonia emanating from the wall or which, if it were not trapped, would emanate from the wall.

The present invention relates to the trapping of ammonia in a pollution-control system intended to reduce the amount of nitrogen oxides in the exhaust gases of a motor vehicle.

The nitrogen oxides present in the exhaust gases of vehicles, in particular diesel vehicles, can be eliminated by a pollution-control system using a technique of selective catalytic reduction (generally referred to as SCR). According to this technique, doses of ammonia (NH₃) are injected into the exhaust line upstream of a catalyst on which the reduction reactions take place.

Currently, the ammonia is produced by thermal decomposition of a precursor, generally an aqueous solution of urea. In the on-board systems for storing urea, the urea solution undergoes, over time, a decomposition reaction to give gaseous ammonia, which reaction increases when the temperature rises. In certain cases, the gaseous ammonia resulting from this decomposition and which is present inside the tank, may pass through the wall of the urea storage tank. Thus, there is a risk of this gaseous ammonia spreading outside of the tank, i.e. into the air surrounding the tank. This is particularly inconvenient.

Indeed, gaseous ammonia is a source of a very acrid odor and is toxic, in particular for man, but also for the environment. Furthermore, it is corrosive for certain metals.

It is therefore necessary to trap the ammonia vapors generated deliberately or accidentally and which are capable of being released from the tank.

From document EP 1 911 508, a device intended to trap the ammonia generated inside a urea storage tank of an SCR pollution-control system is already known. The solution described in that document consists in transporting the gaseous ammonia present inside the urea storage tank to a trap external to the urea storage tank, through a transport pipe. However, the drawback of that solution lies in the fact that there is a risk that a portion of the gaseous ammonia present in the tank is not transported to the trap. This portion of gaseous ammonia may therefore pass through the wall of the tank.

Furthermore, there is also a risk of leakage of ammonia at the transport pipe. However, the aforementioned solution is not suitable for solving this problem. More generally, the solution from the prior art described above does not make it possible to trap ammonia which could escape from components of the SCR system other than the tank, such as for example a urea injection line. The solution from the prior art does not make it possible either to protect components of the SCR system placed in housings, the walls of which may be made of materials that are permeable to ammonia, such as for example a copper coil, more commonly referred to as a winding, of a motor of a urea pump.

The invention aims to trap all or some of the gaseous ammonia generated within an SCR pollution-control system everywhere where it may be inconvenient.

For this purpose, one subject of the invention is a subsystem of a selective catalytic reduction pollution-control system, intended to reduce the amount of nitrogen oxides in the exhaust gases of a motor vehicle, characterized in that said subsystem comprises:

-   -   at least one chamber comprising at least one wall and     -   a trap configured to capture gaseous ammonia emanating from this         at least one wall or which, if it was not trapped, would emanate         from this at least one wall.

A subsystem of a pollution-control system is understood to mean a subassembly of the assembly constituted by all the constituent elements of a pollution-control system that are intended to be built into the vehicle.

A chamber is understood to mean a volume delimited by at least one wall. Thus, in the case where, inside a first volume delimited by a first wall, a second wall delimits a second volume, two chambers are delimited. A chamber corresponds to the space between the two walls. A second chamber corresponds to the second volume delimited by the second wall.

A wall is understood to mean a structure which obstructs (at least partially or completely) the ammonia. More specifically, this structure has an ammonia permeability of less than 3 g/m² per day, for a thickness of 1 mm.

Optionally, the trap is integrated into the wall.

In the invention, the subsystem forms a single block which makes it possible to optimize the overall volume occupied by the chamber and the trap. The assembly of the chamber and of the trap in the pollution-control system is also facilitated by this block configuration.

As a first variant, the chamber contains the trap configured to capture gaseous ammonia and contains an ammonia-sensitive component.

Preferably, the ammonia-sensitive component is made of copper or an alloy thereof, and preferably constitutes a coil of a motor of a urea pump.

Ammonia attacks copper and all the alloys thereof by corrosion. Yet, the on-board systems for storing, dispensing and dosing urea of the SCR pollution-control systems may contain ammonia resulting from the decomposition of the urea and comprise copper-containing components. These components are placed in a housing in order not to be in contact with the urea to which they may also be sensitive. However, the wall of the protective housing is not completely impermeable to ammonia. This configuration of the invention makes it possible to increase the service life of these components, which makes it possible to prevent the occurrence of functional defects in the pollution-control system and confers an economic advantage.

In a second variant, the trap is firmly mounted to the wall in a fixed or removable manner.

The removable nature of the attachment of the trap to the wall makes it possible to replace the trap, for example when it is saturated with ammonia.

The trap may be attached according to various methods such as adhesive bonding, welding, screwing, or other methods.

Advantageously, the subsystem comprises two chambers, a first chamber of which contains an ammonia trap, said two chambers being separated from one another by at least one wall of a second chamber, so that ammonia contained in the second chamber, escaping via permeability or via rupture of said at least one wall, can have no other destination than the first chamber.

Advantageously, the ammonia trap comprises at least one of the following elements:

-   -   a material on which ammonia may be stored by sorption, more         particularly a salt and more particularly still an         alkaline-earth metal chloride such as magnesium chloride, and     -   a superabsorbent polymer.

These elements have a high absorption capacity if they are compared to other absorbent elements such as for example activated carbons. The use of these elements therefore makes it possible to obtain traps of smaller volume.

They also have the advantage of not being dangerous for the environment.

Finally, these elements are characterized by a capacity for trapping ammonia over a relatively long term.

Advantageously, the superabsorbent polymer is in the form of a gel resulting from the absorption of water by said polymer (the ammonia being trapped by this water).

As a variant, the wall of the second chamber, which separates the two chambers, is a wall common to the two chambers.

Optionally, the ammonia trap is in contact with the wall common to the two chambers.

This configuration makes it possible to improve the capturing of the ammonia which is, as soon as it passes the wall separating the two chambers, brought into contact with the trap.

Optionally, the two chambers have two contiguous walls.

Contiguous is understood to mean walls which are side-by-side, separated by a specified distance.

As a variant, the second chamber is suitable for containing an ammonia precursor.

Preferably, the second chamber is suitable for containing urea.

Optionally, the second chamber consists of a urea injection line.

Optionally, the second chamber consists of a urea storage tank.

Advantageously, one wall of at least one of the chambers is made from a thermoplastic material.

As a variant, the first chamber contains an ammonia-sensitive component.

Preferably, the ammonia-sensitive component is made of copper or an alloy thereof, and preferably constitutes a coil or winding of a motor of a urea pump.

Preferably, the second chamber is suitable for containing a compound on which ammonia may be stored by sorption.

Indeed, an alternative technique for provision of ammonia in SCR pollution-control systems consists in storing the ammonia by sorption on a salt, usually an alkaline-earth metal chloride. Generally, in this case, the storage system comprises a tank designed to contain the salt and a heating device configured to heat the salt. Thus, by heating the salt the ammonia is released. The invention also makes it possible to make safe the SCR pollution-control systems having such an ammonia storage device. Indeed, this configuration makes it possible to trap the ammonia which could be released suddenly from the chamber containing gaseous ammonia in the event of an accidental situation such as a defect or a rupture in the wall of this chamber, thus improving the safety of the SCR pollution-control system.

European patent application EP 2 574 599 in the name of the applicant describes an example of a tank intended to store ammonia by sorption on a salt. Said tank comprises a plurality of storage cells that communicate with one another and with at least one orifice that communicates with a dispensing duct. The cells are cavities capable of containing the compound on which the ammonia is stored by sorption.

As a variant, the subsystem comprises a cell, at least one portion of which defines the second chamber.

Optionally, at least one other portion of the cell defines the first chamber.

This embodiment of the invention makes it possible to simplify and accelerate the assembly since the two chambers are provided in the form of a single part. Such a configuration also makes it possible to provide a more compact subsystem.

Advantageously, the two chambers together comprise means for establishing fluid communication which, in the event of overpressure in the second chamber or in the event of desorption operation of the storage means, send the ammonia to the first chamber.

An overpressure in the second chamber may be generated by an excessive heating of the storage means.

Thus, at least one portion of the means for establishing fluid communication, including the connection between the means for establishing fluid communication and the second chamber, is located in the first chamber. This portion consequently also benefits from the safety system formed by the trap.

The means for establishing fluid communication may also be used to discharge the ammonia stored by sorption on the trap, in order to regenerate the latter.

In this configuration, the trap therefore performs three functions.

Advantageously, the trap consists of a matrix which occupies the entire free space of the first chamber.

The space occupied by the matrix increases with the absorption of ammonia. The matrix is then compressed within the volume of the first chamber, thus limiting the flow of ammonia through the wall.

The matrix makes it possible to improve the capturing of ammonia which is, as soon as it passes the wall separating the two chambers, brought into contact with the trap.

The matrix fulfils a thermal insulation role, which makes it possible to prevent the urea solution from reaching too high a temperature in order to limit the release of ammonia due to its decomposition. The thermal insulation also makes it easier to maintain the urea solution at a temperature above its crystallization temperature. Finally, in the event of storage of ammonia by sorption on a salt, the matrix makes it possible to maintain the salt at a stable temperature in order to optimize the control of the release of ammonia by heating.

Another subject of the invention is a selective catalytic reduction pollution-control system comprising a subsystem as described above.

Finally, another subject of the invention is a housing for an ammonia-sensitive component, said housing being intended to be placed in a selective catalytic reduction pollution-control system intended to reduce the amount of nitrogen oxides in the exhaust gases of a motor vehicle, said housing being characterized in that it comprises at least one wall and a trap configured in order to capture gaseous ammonia emanating from this at least one wall or which, if it was not trapped, would emanate from this at least one wall.

All the variants envisaged for the subsystem apply to the housing.

An optional feature of the housing is that it contains an ammonia-sensitive device (for example pH paper), so as to reveal that the trap has been used, which may be one way of easily verifying that the component is still protected at the time of a maintenance operation.

Another optional advantageous feature is the presence of a caulking agent in the wall in order to seal the possible cracks and thus reduce, or even eliminate, leaks of ammonia.

The invention will be better understood on reading the appended figures, which are provided by way of examples and have no limiting nature, in which:

FIG. 1 is a schematic representation of an SCR pollution-control system comprising a urea storage tank.

FIG. 2 is a view of a urea storage tank of an SCR pollution-control system according to a first embodiment of the invention.

FIG. 3 is a view of a urea storage tank of an SCR pollution-control system according to a second embodiment of the invention.

FIG. 4 is a view of a urea storage tank of an SCR pollution-control system according to a third embodiment of the invention.

FIG. 5 is a view of a portion of a urea injection line of an SCR pollution-control system according to a fourth embodiment of the invention.

FIGS. 6 and 7 are views of a urea injection line according to a fifth embodiment of the invention. FIG. 6 is a cross section along IV-IV of FIG. 7 and FIG. 7 is a cross section along V-V of FIG. 6.

FIG. 8 is a cross section of a urea storage tank according to a sixth embodiment.

FIG. 9 is a schematic representation of an SCR pollution-control system comprising a system for storing gaseous ammonia.

FIG. 10 is a view of a system for storing gaseous ammonia according to a seventh embodiment of the invention.

FIG. 11 is a view of a system for storing gaseous ammonia according to an eighth embodiment of the invention.

FIG. 12 is a view of a system for storing gaseous ammonia according to a ninth embodiment of the invention.

In FIGS. 1 to 8, the components which are identical are denoted by the same reference numbers.

Reference is now made to FIG. 1, in which a system is represented for treating nitrogen oxides present in the exhaust line 2 of a vehicle engine 1. The nitrogen oxides are sent to a catalyst 8 in which the selective catalytic reduction (SCR) is carried out. The selective catalytic reduction is obtained by addition of ammonia to the exhaust gases. In the example from FIG. 1, the ammonia needed for the reduction originates from a urea solution 4 which is stored in a urea storage tank 3. The urea storage tank 3 is connected to the exhaust line 2 by a urea injection line 5. The urea present in the tank 3 is transported to the urea injection line 5 owing to the action of a urea pump 6 present inside the urea storage tank 3. Under the action of a urea injector 7, the urea is injected into the exhaust line 2. With time and temperature variations, a portion of the urea contained in the tank 3 is decomposed to give gaseous ammonia. The gaseous ammonia of the tank is capable of coming into contact with ammonia-sensitive components present in the urea storage tank 3.

A first embodiment of the invention has been represented in FIG. 2. A urea storage tank 3 contains an ammonia-sensitive component 9. This component is placed in a housing 10 delimited by a wall 17 and a cover 11. Since the wall 17 of the housing and the cover 11 are porous to ammonia, an ammonia trap 12 was placed in the wall 17 of the housing and that of the cover 11 in order to keep, in the housing, the concentration of ammonia below a threshold value capable of leading to the corrosion of the component. This threshold value may be determined as a function of the nature of the component, of the temperature or of the exposure time. This threshold value or these threshold values may be obtained following experiments. In this example, the subsystem consists of the assembly of the housing 10 which corresponds to the chamber comprising two walls, namely the wall 17 of the housing and the cover 11. The trap 12 is configured in order to capture gaseous ammonia emanating from the wall 17 or from the cover 11. Alternatively, the trap may be placed in one or other of the walls of the housing and of the cover. In this embodiment, the trap 12 is in the form of magnesium chloride particles integrated into the polymer wall 17 of the housing and of the cover 11. In one particular embodiment, the wall 17 of the housing has, for example, a thickness of around 2 mm and has a permeability equal to around 1.25 g/m²/day at 80° C. In this particular embodiment, the housing 10 has, for example, a total surface area of around 300 cm². In this embodiment, the protection of the component is ensured by 3 to 6 g of magnesium chloride.

A second embodiment of the invention has been represented in FIG. 3. An ammonia-sensitive component 9′ is placed in a housing 10′ equipped with a cover 11′ and placed in the vicinity of a urea storage tank 3 delimited by a wall 18. The walls 17′ of the housing and 18 of the urea storage tank 3 may be a single wall common to the two chambers. Since these two walls are porous to ammonia, an ammonia trap 12′ was placed inside the tank 3, in the vicinity of the location of the housing 10′ in order to keep, in the housing, the concentration of ammonia below a threshold value capable of leading to the corrosion of the component. This threshold value may be determined as a function of the nature of the component, of the temperature or of the exposure time. This threshold value or these threshold values may be obtained following experiments. In this example, the subsystem consists of the assembly of the urea storage tank 3 which corresponds to the chamber comprising the wall 18. The trap 12′ is configured to capture gaseous ammonia which, if it was not trapped, would emanate from the wall 18. The trap is provided with a protection 19 that limits direct contact between the ammonia trap and the liquid phase of the ammonia precursor. In this embodiment, the trap 12′ is firmly mounted to the wall 18. In this embodiment, the protection of the component is ensured, for example, by 1 to 2 g of magnesium chloride. In one particular embodiment, the walls 17′ of the housing 10′ and 18 of the urea storage tank 3, have a total thickness of 2 mm and a permeability equal to 1.25 g/m²/day at 80° C. This common wall has, for example, a surface area of 100 cm², or in the case of two superposed walls, these may be superposed over a surface area of 100 cm².

A third embodiment of the invention has been represented in FIG. 4. A urea storage tank 3 contains an ammonia-sensitive component 9″. This component is placed in a housing 10″ having a wall 17″ and equipped with a cover 11″. Since the wall 17″ of the housing and the cover 11″ are porous to ammonia, an ammonia trap 12″ was placed in the cover 11″ of the housing in order to keep, in the housing, the concentration of ammonia below a threshold value capable of leading to the corrosion of the component. This threshold value may be determined as a function of the nature of the component, of the temperature or of the exposure time. This threshold value or these threshold values may be obtained following experiments. In this example, the subsystem consists of the assembly of the housing 10″ which corresponds to the first chamber and of the urea storage tank 3 which corresponds to the second chamber. The two chambers are separated from one another by the wall 17″ of the housing 10″ which corresponds to a wall of the second chamber. In this embodiment, the trap 12″ is in the form of a pad composed of an open-cell foam made for example of polyethylene, impregnated with magnesium chloride. In one particular embodiment, the wall of the housing 10″ has, for example, a thickness of around 1 mm and has a permeability equal to around 2.5 g/m²/day at 80° C. In this particular embodiment, the housing 10″ has, for example, a total surface area of around 300 cm². In this embodiment, the protection of the component is ensured by 6 to 12 g of magnesium chloride.

A fourth embodiment of the invention has been represented in FIG. 5. A urea injection line 5 contains an ammonia-sensitive component 9″. As in the preceding embodiment, the component 9″' is placed in a housing 10′″ having a wall 17″′, equipped with a cover 11″′. An ammonia trap 12′″ was placed in the cover of the housing. In this example, the subsystem consists of the assembly of the housing 10″′ which corresponds to the first chamber and of the urea injection line 5 which corresponds to the second chamber. The two chambers are separated from one another by the wall 17′″ of the housing 10″′ which corresponds to a wall of the second chamber. The trap 12″′ is in the form of a pad composed of an open-cell foam made for example of polyethylene, impregnated with magnesium chloride. In one particular embodiment, the wall 17″′ of the housing 10″′ consists of polyphthalamide (PPA), has, for example, a thickness of around 1 mm and has an ammonia permeability at 80° C. of around 1 to 2.5 g/m²/day. In this particular embodiment, the housing 10′″ has, for example, a total surface area of around 300 cm². In this embodiment, the protection of the component over 40 days (around 1000 hours) is ensured by 2 to 6 g of magnesium chloride.

A fifth embodiment of the invention has been represented in FIGS. 6 and 7. A urea injection line 5 is surrounded over one portion of its length by a sleeve 13 containing an ammonia trap 14. In this example, the subsystem consists of the assembly of the sleeve 13 which corresponds to the first chamber and of the urea injection line 5 which corresponds to the second chamber. The two chambers are separated from one another by the wall of the urea injection line 5 which corresponds to the wall of the second chamber. In this embodiment, the trap consists of magnesium chloride crystals. In one particular embodiment, the wall consists of polyamide 66 (PA 66) and has, for example, an ammonia permeability of around 0.5 g/m²/day at 40° C. for a thickness of around 1 mm. In this particular embodiment, for a urea injection line of around 4 mm in diameter and of around 1 m in length, the surface area covered is around 125 cm². In this embodiment, around 0.25 g of ammonia is emitted in 40 days (around 1000 hours) through the wall of the urea injection line. Around 0.25 g of magnesium chloride may ensure the trapping of this amount of ammonia. In this particular embodiment, the amount of magnesium chloride forming the trap in such an embodiment is around 0.5 to 2 g.

A sixth embodiment has been represented in FIG. 8. A urea storage tank 3 is surrounded by a shell 15 containing an ammonia trap 16. In this example, the subsystem consists of the assembly of the shell 15 which corresponds to the first chamber and of the urea storage tank 3 which corresponds to the second chamber. The two chambers are separated from one another by the wall of the urea storage tank 3 which corresponds to the wall of the second chamber. In this embodiment, the trap consists of a magnesium chloride matrix. In one particular embodiment, the wall consists of high-density polyethylene (hdPE) and has, for example, an ammonia permeability at 80° C. of around 1.5 g/m²/day for a thickness of around 1 mm. In one particular embodiment, for a thickness of around 4 mm and a tank surface area of around 1 m², the protection over 40 days (around 1000 hours) may be ensured, for example, by 14 g of magnesium chloride. In practice, in this embodiment, the trap contains, for example, 15 to 25 g of magnesium chloride.

In FIGS. 9 to 12, the components which are identical are denoted by the same reference numbers.

Represented schematically in FIG. 9 is an example of a selective catalytic reduction (SCR) pollution-control system comprising a system for storing gaseous ammonia. The invention is not limited to such an example of an SCR system with gaseous storage. The engine 21 of the vehicle is controlled by an electronic control unit 22. The engine 21 cooperates with an SCR system 23. On leaving the engine, the exhaust gases 41 are sent to an ammonia injection module 31, in which the ammonia 72 is mixed with the exhaust gases 41. The ammonia/exhaust gas mixture 43 then passes through an SCR catalyst 32 which enables the reduction of the nitrogen oxides (NOx) by the ammonia. The decontaminated exhaust gases 44 are then sent to the exhaust outlet.

The SCR system 23 comprises an ammonia storage system 25. The storage system 25 comprises a cell 54 in which a compound 52, for example a solid (preferably a salt), is stored. The ammonia is stored by sorption on the solid 52. The storage system 25 also comprises a control device 24 in charge of controlling a heating device 53 for heating the solid 52 so as to release the ammonia. The cell 54 is connected to a dosing module 51, via a dispensing duct 27. The dosing module 51 is controlled by the control device 24. The control device 24 is capable of estimating the pressure of ammonia in the storage system 25. If a difference is observed between the estimated pressure and a setpoint pressure provided by the electronic control unit 22, the control device 24 may adjust the heating power of the heating device 53 in order to compensate for this difference. The tank 54 is equipped with a temperature-measuring device 26.

Represented in FIG. 10 is a seventh embodiment of the invention in a selective catalytic reduction (SCR) pollution-control system comprising a system for storing gaseous ammonia. A cell 54, which contains a compound 52 on which the ammonia is stored by sorption, is surrounded by a shell 63 which delimits a chamber according to the invention that contains an ammonia trap 62. A dispensing duct 27 connected to the cell 54 leads to a three-way valve 60. In the event of overpressure in the cell 54 and of no request by the control device 24 for a discharge of ammonia to the component located downstream of the cell 54, namely, in this example, the temperature-measuring device 26, the excess ammonia is rerouted to a duct 61 which is connected to the shell 63 that contains the trap 62. In this example, the subsystem consists of the assembly of the shell 63 which corresponds to the first chamber and of the cell 54 which corresponds to the second chamber. The shell 63 and the cell 54 are separated from one another by the wall of the cell 54 which corresponds to the wall of the second chamber. The dispensing duct 27, the three-way valve 60 and the duct 61 constitute means for establishing fluid communication within the meaning of the invention. The three functions already mentioned for the trap are found: making safe the second chamber and a portion of the means for establishing fluid communication, and also the control of the overpressure and of the desorption. In this embodiment, the cell 54 has a volume of 500 ml. The trap 62 consists of a magnesium chloride matrix.

In one particular embodiment, the thermal activation of the desorption results in an emission of around 1 g of ammonia, which translates into an overpressure of around 4 bar in the cell 54. When the system is closed, all of the desorbed ammonia is sent to the trap. For example, 0.93 g of magnesium chloride is needed for trapping around 1 g of ammonia. Nevertheless, it is for example advantageous to provide the trap with 5 g of magnesium chloride in order to effectively absorb all of the ammonia suddenly discharged.

In the event of an accidental situation such as a defect or a rupture in the wall of the cell 54, at a temperature of 40° C., the desorption of the ammonia takes place with a flow rate of around 3.5 mg/s, i.e. around 12.6 g of ammonia in 1 hour. In this particular embodiment, around 11.9 g of magnesium chloride are needed to absorb this amount of ammonia. For example, a trap containing 12 g of magnesium chloride makes it possible to ensure the protection with respect to such an accidental situation for around 1 hour.

Finally, if it is intended to trap the ammonia resulting from a desorption over a long duration, that is to say all the ammonia contained in the cell 54, a trap containing for example 300 g of magnesium chloride is needed.

Represented in FIG. 11 is an eighth embodiment of the invention in a selective catalytic reduction (SCR) pollution-control system comprising a system for storing gaseous ammonia. Three identical cells 54 a, 54 b and 54 c, of 500 ml each, are all three surrounded by a same shall 59 which contains an ammonia trap 58. In this example, three subsystems are considered. The first consists of the assembly of the shell 59 which corresponds to the first chamber and of the cell 54 a which corresponds to the second chamber. The second consists of the assembly of the shell 59 which corresponds to the first chamber and of the cell 54 b which corresponds to the second chamber. The third consists of the assembly of the shell 59 which corresponds to the first chamber and of the cell 54 c which corresponds to the second chamber. The shell 59 is separated from the cells 54 a, 54 b and 54 c by the walls of the latter which correspond, for each subsystem, to the wall of the second chamber. A dispensing duct 27 connected to the cells 54 a, 54 b and 54 c is equipped with a pressure relief valve 56. In the event of overpressure in the cells 54 a, 54 b and 54 c and of no request by the control device 24 for a discharge of ammonia to the component located downstream of the cells 54 a, 54 b and 54 c, namely, in this example, the temperature-measuring device 26, the excess ammonia is rerouted to a duct 57 which is connected to the shell 59 that contains the trap 58. The dispensing duct 27, the pressure relief valve 56 and the duct 57 constitute means for establishing fluid communication within the meaning of the invention. As for the preceding figure, the three functions of the trap are found. The trap 58 consists of a magnesium chloride matrix.

In one particular embodiment, the thermal activation of the desorption results in an emission of around 1 g of ammonia in each cell 54 a, 54 b and 54 c, which translates into an overpressure of around 4000 hPa in each cell. When the system is closed, all of the desorbed ammonia is sent to the trap. Around 2.79 g of magnesium chloride are needed for trapping around 3 g of ammonia resulting from the three cells. Nevertheless, it is advantageous to provide the trap for example with 15 g of magnesium chloride in order to effectively absorb all of the ammonia suddenly discharged.

In the event of an accidental situation such as a defect or a rupture in the wall of one of the cells, at a temperature of 40° C., the desorption of the ammonia takes place with a flow rate of around 3.5 mg/s, i.e. around 12.6 g of ammonia in 1 hour. For example, around 11.9 g of magnesium chloride are needed to absorb this amount of ammonia. A trap containing around 12 g of magnesium chloride makes it possible for example to ensure the protection with respect to such an accidental situation for around 1 hour.

Finally, if it is intended to trap the ammonia resulting from a desorption over a long duration, that is to say all the ammonia contained in the cells 54 a, 54 b and 54 c, a trap containing for example 900 g of magnesium chloride is needed.

Represented in FIG. 12 is a ninth embodiment of the invention in a selective catalytic reduction (SCR) pollution-control system comprising a system for storing gaseous ammonia. A cell 70 contains two compartments 74 and 73 separated from one another by a partition 71. The compartment 74 contains a compound 52 on which the ammonia is stored by sorption. A dispensing duct 27 connected to the compartment 74 leads to a three-way valve 60. In the event of overpressure in the compartment 74 and of no request by the control device for a discharge of ammonia to the component located downstream of the container 70, in this example, the temperature-measuring device 26, the excess ammonia is rerouted to a duct 61 which is connected to the compartment 73 that contains a trap 62. In this example, the subsystem consists of the assembly of the compartment 73 which corresponds to the first chamber and of the compartment 74 which corresponds to the second chamber. As for the preceding two figures the three functions of the trap are found, although, in this example, the wall separating the two chambers does not entirely surround the other chamber. In this embodiment, the compartment 74 has a volume of 500 ml. The trap 62 consists of a magnesium chloride matrix.

In one particular embodiment, the thermal activation of the desorption results in an emission of around 1 g of ammonia, which translates into an overpressure of around 4000 hPa in the compartment 74. When the system is closed, all of the desorbed ammonia is sent to the trap. For example, 0.93 g of magnesium chloride is needed for trapping around 1 g of ammonia. Nevertheless, it is advantageous to provide the trap for example with 5 g of magnesium chloride in order to effectively absorb all of the ammonia suddenly discharged.

In the event of an accidental situation such as a defect or a rupture in the partition 71, at a temperature of 40° C., the desorption of the ammonia takes place with a flow rate of around 3.5 mg/s, i.e. around 12.6 g of ammonia in 1 hour. For example, 11.9 g of magnesium chloride are needed to absorb this amount of ammonia. A trap containing around 12 g of magnesium chloride makes it possible to ensure the protection with respect to such an accidental situation for around 1 hour.

Finally, if it is intended to trap the ammonia resulting from a desorption over a long duration, that is to say all the ammonia contained in the compartment 74, a trap containing around 300 g of magnesium chloride is needed.

The invention is not limited to the embodiments presented and other embodiments will appear clearly to a person skilled in the art. In particular it is possible to modify the first two embodiments represented in FIGS. 2 and 3 in order to add at least a second trap to a portion of the housing located opposite the first trap.

The embodiments may also be modified in the form of the trap (matrix, salt in crystalline form, etc.). The trap may contain another salt such as, for example, strontium chloride or calcium chloride. It may alternatively consist of water-saturated superabsorbent polymers. 

1-10. (canceled)
 11. A housing for an ammonia-sensitive component, the housing configured to be placed in a selective catalytic reduction pollution-control system configured to reduce an amount of nitrogen oxides in exhaust gases of a motor vehicle, the housing comprising: at least one wall and a trap configured to capture gaseous ammonia emanating from the at least one wall or which, if it was not trapped, would emanate from this at least one wall.
 12. The housing as claimed in claim 11, wherein the trap is integrated into the wall.
 13. The housing as claimed in claim 11, wherein the trap is firmly mounted to the wall in a fixed or removable manner.
 14. The housing as claimed in claim 11, wherein the ammonia trap comprises at least one of the following elements: a material on which ammonia may be stored by sorption, or a salt or an alkaline-earth metal chloride or magnesium chloride; or a superabsorbent polymer.
 15. The housing as claimed in claim 14, wherein the superabsorbent polymer is in a form of a gel resulting from absorption of water by the polymer.
 16. The housing as claimed in claim 11, further comprising an ammonia-sensitive device.
 17. The housing as claimed in claim 11, further comprising a caulking agent in the wall to seal possible cracks and reduce or eliminate leaks of ammonia.
 18. The housing as claimed in claim 11, wherein the ammonia-sensitive component includes a winding of a motor of a urea pump.
 19. The housing as claimed in claim 11, placed in a urea storage tank of the selective catalytic reduction pollution-control system.
 20. The housing as claimed in claim 11, is placed in a urea injection line of the selective catalytic reduction pollution-control system. 